Fluid compositions are coated onto substrates for use in a wide variety of applications. Some of those applications require the compositions to be coated very thinly on the substrates; many of such compositions are formed into high viscosity fluids such as dispersions or emulsions. In order to process dispersions, emulsions and other high viscosity coatings, the processing system must provide high uniform energy to disperse the agglomerate structure in such materials. Pulsation, or the application of non-uniform energy, will result in some of the product experiencing insufficient processing, which cannot be cured by increasing the processing time. Such pulsations in fluid processing systems have been addressed by the development of high intensifier shear devices, such as those disclosed in U.S. Pat. No. 6,558,134. However, after the processing/shearing of the fluid is completed, the fluid must then be delivered to a coating device, for example, a coating die.
Current dispersion delivery systems utilize a gear pump to meter the dispersion to the coating device. Balancing the metering of the flow can be very difficult, especially in low flow situations. Pressure fluctuations, or pulsations caused by the gear pump(s), can lead to undesirable coating caliper variations, especially in applications requiring precise and high pressure delivery of thin coatings. For thin coatings, the coating calipers are small. This means that very small amounts of the dispersion are delivered to the coating device at a time; i.e., the delivery systems have very low flow. This low flow lengthens the waiting time between the shearing step and the coating steps, which can cause degradation of sheared material. The low flow situation also makes control of the coating thickness and weight more difficult. One problem can occur when such small amounts are involved. Normal pressure variations in the delivery system pumps result in significant variations in the amount of the fluid provided to the coating die. These variations in the fluid delivery can result in significant variations in the caliper of the coatings, and thus unacceptable defects.
Various dispersion delivery systems have been proposed or attempted to control the coating thickness, to varying degrees of success, and sometimes creating other problems. One system to improve the flow, while still balancing the metering is disclosed in EP 0435351 A1. This delivery system uses gear pumps which provide metering into the system of excess flow which goes to the coating die, followed by metering out the excess flow at or just prior to contact with the coating web.
One specific application for thin precise coating(s) on a substrate is for use as magnetic recording layers or support layers for magnetic recording layers. Such formulations are typically magnetic or electroconductive or other particles dispersed in binder systems with various adjuvants, such as lubricants and head cleaning agents (HCA). Variations in coating caliper for magnetic recording media adversely affect the data quality delivered of the recording media. The formulations, and dispersions made therefrom, are proprietary to the magnetic recording manufacturers, and vary with the type and variation of magnetic media produced. However, the density of recording in magnetic recording media has become higher in recent years, requiring higher quality formulations and coatings.
It would be desirable to have a dispersion delivery system which, when used in conjunction with a high intensifier shear system, would provide consistent amounts of material to the coating device and thus reduce variations in the subsequent coatings. It would also be advantageous if such dispersion delivery system did not include a gear pump which could introduce such pressure, and thus caliper fluctuations.
It has now been discovered that a pressure-controlling dispersion delivery system used in combination with a shear device, preferably a high intensifier shear system, will result in consistent delivery of high pressure fluid to a coating device, even in a low flow system. Pressure-controlling dispersion delivery devices can be provided using proportional integral delivery (PID) control loop.